In order to increase the efficiency of gas turbines, jet engines, and the like, the temperature of the combustion gas continues to be increased. As a result, in order to protect metal parts from the high temperature (for example, blade surface temperature of a 1500° C. class gas turbine reaches approximately 1400° C.), a thermal barrier coating (TBC) is coated on the surface of the parts. As a material for this thermal barrier coating, ceramic having a low thermal conductivity such as rare earth stabilized zirconia is used (for example, refer to Japanese Unexamined Patent Applications, First Publications No. Hei 08-074505, and No. Hei 10-183013). The above thermal barrier coating is applied by means of atmospheric pressure plasma spraying over a metallic adhering layer applied to a substrate, which is the metal part, by means of low pressure plasma spraying or the like.
The thermal barrier coating applied on the metal part by means of atmospheric pressure plasma spraying is not of a dense constitution, and internally has a large number of pores. FIG. 1 shows a schematic diagram of the constitution of the thermal barrier coating. As shown in FIG. 1, the structure of the thermal barrier coating is such that a zirconia substrate 1 has various kinds of pores such as large pores 2 with diameters of several tens of microns, small pores 3 with diameters of several microns, and narrow, line-shaped pores 4 and 5. The zirconia substrate 1 itself is a ceramic having a low thermal conductivity, and at the same time, a large number of such pores 2 to 5 inside maintain the thermal insulation property of the material. As a result, the substrate, which is the metal part, can be used in a high temperature environment.
Zirconia, which is used as a material for high temperature structures including thermal barrier coatings, is not a single composition (ZrO2), but is used in a state in which several mol % of rare earth oxide are added as a stabilizing agent (partially stabilized zirconia). The reason for this is that pure Zirconia (ZrO2), to which no stabilizer has been added, has the following two phase transitions:Monoclinic←up to 1000° C.→tetragonal←2370° C.→cubic
and the volume of pure zirconia itself changes acutely as its temperature rises or drops, with the result that it is destroyed in the phase transition between monoclinic and tetragonal phases, so that pure zirconia cannot be used as a material for high temperature structure. Consequently, it is necessary to add several mol % of rare earth oxide and to stabilize the tetragonal phase, which is the phase in the working temperature range, even at low temperature, so as not to generate a monoclinic phase. It has been reported that even for partially stabilized zirconia, for which the amount of stabilizing agent is controlled in order to stabilize the tetragonal phase, a monoclinic phase progressively precipitates when used for a long period of time at a high temperature and in a heat cycle where the temperature is repeatedly increased and decreased, and this has been an important problem in using zirconia as a thermal barrier coating.
There has also been a report of applying material of a cubic pyrochlore type structure such as La2Zr2O7 as a thermal barrier coating material instead of zirconia (refer to Japanese Unexamined Patent Application, First Publication No. Hei 10-212108, European Patent No. 0848077, and U.S. Pat. No. 6,117,560). In these patent documents, La2Zr2O7 is a suitable material for a thermal barrier coating because its thermal conductivity and oxygen permeability are smaller than that of zirconia. However, there is a problem of an actual remaining tensile stress between the coating and the substrate metal part, since the thermal expansion coefficient of La2Zr2O7 is smaller than that of zirconia.